The cell biology studies have been focused on: (1) the role of human T-lymphotropic retroviruses (HTLV) in human T-cell malignancies and acquired immunodeficiency syndrome (AIDS), and (2) a B-lymphotropic DNA virus (HBLV). HTLV-I has been shown to be a transforming virus, whereas human immunodeficiency virus (HIV-1) is cytopathic. HTLV-I, HTLV-II, and HIV-1 have specificity for OKT4 positive T helper cells. The involvement of these viruses in neuropathy is being examined. HTLV-1 has recently been isolated from patients with tropical spastic paraparesis (TSP). HIV-1 has been shown to be associated with cells of monocyte-macrophage lineage. HIV-1 isolates obtained from different patients show some genetic variations in the envelope region. Drugs that block HIV-1 replication are being tested in in vitro systems. Studies in chimpanzees show the development of antibodies against HIV-1 antigens and viremia in these animals on inoculation with HIV-1. Vaccine studies indicate the development of neutralizing antibodies against the virus envelope, and more recently antibody against a synthetic HIV-1 p17 peptide (HGP30) was found to block HIV-1 replication in cell culture. The worldwide distribution of HTLV infection and the mechanism of its transmission in patients with AIDS and AIDS- related complex (ARC) have been extensively studied.